As one microlens, a rod lens in which both end faces of a columnar body that can be made of glass or a plastic material are mirror polished has been known. In addition to being used individually, rod lenses are also used in the form of a rod lens array in which a plurality of rod lenses are adhesively fixed to be arranged in parallel in one line or a plurality of lines.
Such rod lens arrays are widely employed as optical devices for reading for image sensors installed in various scanners such as hand scanners, or photocopying machines, facsimile machines, etc.
Photocopying machines have been advancing in colorization; therefore, the optical performance adapted to the reading of color images has been demanded also in rod lens arrays, which are optical devices for the built-in image sensors.
Then, in recent years, models that handle color images have been developed also for compact scanners such as hand scanners and flathead-type scanners, and also in such scanners, optical performance adapted to the reading of color images has been demanded in rod lens arrays, which are optical devices for built-in image sensors.
In order to respond to such demand, for example, a rod lens excelling in color characteristics has been developed that can obtain a color image with little chromatic aberration and little bleeding (Patent Document 1).
However, hand scanners, flathead-type scanners used in photocopying machines, etc. differ from facsimile machines, sheet-feed type scanners, etc., and the original document face is not fixed; therefore, the original floats during image reading, the distance between the lens surface of the rod lens and the original document face varies, and focusing the image may not be possible.
For this reason, for hand scanners, flathead-type scanners, etc. used in photocopying machines and the like, rod lenses of deep depth of focus have been demanded so that a clear image is obtained even if the distance between the lens face of the rod lens and the original document face somewhat varies.
Furthermore, in addition to colorization of images, the miniaturization of devices is advancing, and thus rod lenses that can realize compact optical systems have been demanded.
Generally, since the depth of focus and numerical aperture of lenses are in an inverse proportional relationship, it is possible to obtain a rod lens of deep depth of focus by decreasing the numerical aperture. However, since the conjugation length becomes longer as the numerical aperture decreases, it becomes difficult to achieve miniaturization of the optical system.
In order to deal with such a situation, Patent Document 2 discloses a rod lens that reduces the numerical aperture by decreasing the effective diameter of the lens by providing a light absorption layer having a thickness of at least 50 μm at the lens outer periphery, thereby shortening the conjugation length, and deepening the depth of focus.
Furthermore, Patent Document 3 discloses a rod lens with deepened depth of focus by setting a refractive index distribution constant g of the rod lens to 0.2 mm−1≦g≦0.325 mm−1, and setting the product of g and the rod lens radius r to 0.04≦g·r<0.065.
[Patent Document 1] Pamphlet of PCT International Publication No. WO2007/011013
[Patent Document 2] Japanese Unexamined Patent Application, Publication No. 2000-35519
[Patent Document 3] Japanese Unexamined Patent Application, Publication No. 2003-139912